1. Field of the Invention
The present invention relates to an optical system that is used in an image pickup apparatus or the like. More particularly, the present invention relates to a wide angle optical system that is used in a digital single lens camera or a compact camera and also to an image pickup apparatus using the same.
2. Description of the Related Art
Wide angle lenses of digital cameras require an optical system having a long back focal length because they need a space for receiving optical elements including a filter immediately in front of the electronic imaging plane, while securing a wide angle of view. Retrofocus type optical systems having a front lens group of negative refractive power and a back lens group of positive refractive power are being popularly employed for wide angle lenses.
When an incident flux of light that strikes the electronic imaging plane of a wide angle lens obliquely relative to the optical axis after being transmitted through the optical system to give rise to an obliquely incident state, the peripheral quantity of light is reduced to produce shading and color misregistration. Therefore, optical systems keeping telecentricity for the incident flux of light have been proposed. Techniques of this type include those disclosed in Patent Document 1 (JP-A-2000-235145), Patent Document 2 (JP-A-11-249009), Patent Document 3 (JP-A-11-142730), Patent Document 4 (JP-A-09-166748) and Patent Document 5 (JP-A-11-242155).
There has been in recent years and still is a demand for compact wide angle lenses having a reduced number of component lenses and a reduced total lens length to be used in digital cameras. It is difficult to design an optical system for a lens with a small number of component lenses, while securing a wide angle of view, suppressing various aberrations and maintaining the telecentricity at the same time. However, due to the technological development in recent years in the field of electronic image pickup elements, it is no longer necessary to strongly pay attention to correction of distortions as techniques have been proposed to correct distortions to a certain extent by means of electric signals.
In view of the above-described circumstances, it is therefore an object of the present invention to provide a compact wide angle optical system of an angle of view of about 60° that has a small number of component lenses and can correct various aberrations, such as chromatic aberration and field curvature in particular, while performing correction of distortions to a certain extent and secure telecentricity at the same time.